The present Application claims the benefit of Japanese Patent Application No. 2001-265-461, filed Sep. 3, 2001, the contents of which are incorporated in this disclosure by reference in its entirety.
The present invention relates to a method of treating the surface of Ti alloy.
Ti provides high specific strength but is likely to be worn. So surface treatment is required.
To make surface treatment to a Ti alloy poppet valve, there are thermal spray, Ni plating, nitriding in Japanese Patent Laid-Open Pub. No. 61-81505, oxidation in Japanese Patent Laid-Open Pub. Nos. 62-256956 and 3-36257, and plasma carburizing in Japanese Patent No. 2909361.
In thermal spray and plating, to remove an oxide film which is likely to be formed on the surface and to increase adhesion of hard material, the surface of a workpiece must be roughened by shot blasting and pretreated with acid washing to make the process more complicate to increase cost. Hard films are likely to come off.
In nitriding and oxidation, the workpiece is heated, which is relatively simple, but the surface is so hard as to increase offensiveness to an opposite member such as a valve seat and valve guide, which must be replaced in material to increase cost.
In oxidation, a workpiece is heated in oxygen excessive atmosphere to increase oxygen diffusion speed and to form a relatively thick fragile oxide film such as TiO2 and Ti2O3, which is likely come off. So the oxide film must be removed by shot blasting or machining until an oxygen diffusion layer appears, thereby increasing cost.
In plasma carburizing, wear resistance required for a poppet valve is available, but it is necessary to provide expensive equipment such as vacuum furnace and plasma electric source to increase depreciation and running cost which causes increased unit price.
It is an object of the present invention to provide a method of treating the surface of Ti alloy to increase wear resistance at low cost and simple means.
In order to achieve the object, according to the present invention, there is provided a method of treating the surface of Ti alloy, comprising the steps of embedding the Ti alloy in oxygen-absorptive powder, and heating said Ti alloy with the powder in oxygen atmosphere to diffuse oxygen atoms into the Ti alloy to form an oxygen diffusion layer of Tixe2x80x94O solid solution.
Without forming an oxide layer on the surface of Ti alloy, a hard oxygen diffusion layer can be formed to provide a wear-resistant Ti alloy.